Ecological awareness about such things as using clean energy is not limited to academic and industrial areas, and has now reached ordinary consumers. In the midst of this, there has been a tremendous amount of development going into hybrid vehicles and fuel cell vehicles, and consumers now have a heightened purchase awareness. Ensuring the safety of these hybrid vehicles and fuel cell vehicles is essential, and it is important to inspect and maintain the containers that hold high-pressure hydrogen for fuel cells. For example, with a hydrogen fuel cell vehicle that makes use of high-pressure hydrogen, a high-pressure tank that holds the high-pressure hydrogen is installed in the fuel cell vehicle (hereinafter referred to as a hydrogen cell vehicle).
In general, these high-pressure tanks used for hydrogen fuel cells contain high-pressure hydrogen gas of 35 MPa. When a high-pressure tank is repeatedly filled with this high-pressure hydrogen gas, microcracks occur in the high-pressure tank, and these can spread out and lead to the failure of the high-pressure tank. Thus, to ensure that a high-pressure tank is safe, it is important that the failure, and particularly signs of failure, in a high-pressure tank be detected early and reliably.
A test method for detecting microscopic defects (cracks or voids) inside or on the surface of a material without physically destroying the test sample is called non-destructive testing. Non-destructive testing includes radiographic testing, ultrasonic testing, and so forth. Using acoustic emissions (hereinafter referred to as AE) is another test method used in non-destructive testing. With AE, it is possible to detect the first sign of cracking, and this is used particularly for monitoring cracking during the operation of equipment or how far the cracks have proceeded.
Event Method and Ring Down Method
First, let us describe AE measurement and its processing. AE is an elastic wave produced when cracks form and spread in a material. One AE signal is made up of elastic waves of a plurality of frequencies generated continuously in a short period, and the size and strength thereof vary with the size of the crack.
The following are two methods for processing AE signals received by an acoustic emission sensor (hereinafter referred to as an AE sensor). The first method is the event method, in which one AE signal is counted as one. With this event method, the AE signals being counted are called AE hits, and the number of AE hits per unit of time is called the AE hit rate. This AE hit rate is routinely used to evaluate the spread of fatigue cracks, taking into account the fact that AE signals generated from cracks that spread due repeated stress are basically discrete.
The second method is the ring down method, in which all amplitudes of a defined reference value or greater are counted. An AE signal counted by this ring down method is called an AE count, and the number of AE counts per unit of time is called the AE count rate. FIGS. 14(a), 14(b) and 14(c) illustrate the differences between the event method and the ring down method. FIG. 14(a) shows one AE signal. FIGS. 14(b) and 14(c) show the differences between the event method and the ring down method, which are two methods for counting AE signals.
FIG. 14(b) illustrates the event method. FIG. 14(c) illustrates the ring down method. The maximum amplitude of the AE signal in FIG. 14(a) is at least a set threshold. As shown in FIG. 14(b), the AE signal in FIG. 14(a) is counted as “1” with the event method. With the ring down method, all of the elastic waves that make up a single AE signal and are at or above the set threshold are counted. Therefore, as shown in FIG. 14(c), the AE signal in FIG. 14(a) is counted as “4” with the ring down method.
Many non-destructive testing methods that involve the use of AE signals have been proposed. For example, Patent Document 1 discloses a predictive method for determining the breaking load of a tank or other structure in non-destructive testing of the structure with acoustic emission. This predictive method involves counting the number of hits for AE energy generated in the process of destroying a tank, and predicting the breaking load on the basis of the total count. In other words, the integrated value for energy is used to determine the predicted value of the breaking load.
Patent Document 2 discloses a tank testing apparatus that assesses an area that has been damaged by corrosion at the bottom of a metal tank that holds a liquid or gas. This tank testing apparatus uses an AE sensor to assess an area where corrosion damage has occurred. Time-frequency conversion is performed at various time points on the detected waveform, and the signal amplitude for each frequency band is found as a time series. Consequently, information is obtained about the time of reaching a wave of a specific mode at a specific frequency, making it possible to determine the sound source to high precision, for example (see paragraph [0015] in Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-Open No. H8-54330    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-17089
However, while the method described in Patent Document 1 does allow the static breaking load of a structure to be predicted, no mention is made of structural fatigue failure that is caused by repeated pressure exertions under the static breaking load. That is, although the method described in Patent Document 1 does allow the static breaking load of a high-pressure tank or the like to be predicted, the fatigue life cannot be predicted.
With the method in Patent Document 2, it is possible to detect the position where countless microcracks prior to failure have grown into macroscopic cracks in a relatively small vessel such as a hydrogen tank to be mounted in a vehicle, but it is not possible to obtain an accurate sign of failure before the cracks become macroscopic and lead to failure.